1. Field of the Invention
The present invention relates to a press-contact type semiconductor device, more particularly, relates to MOS gate drive switching devices such as IGBT (Insulated Gate Bipolar Transistor) having a plurality of semiconductor substrates.
2. Description of the Related Art
Conventionally, the structure of the press-contact type semiconductor device has been only one which press-contacts a single semiconductor substrate (hereinafter referred to as "chip"). For example, in the press-contact type device of the anode short type GTO (Gate Turn-Off) thyristor, the disc type chip includes N-type emitter layer, P-type base layer, N-type base layer and P-type emitter layer, from the chip surface which is the cathode toward the back face of the chip which is the anode.
N-type emitter region on the chip surface is formed as a plurality of mesas on P-type base layer. These N-type emitter regions has an aluminum cathode electrode on the top portion of said mesas, and a plurality of gate electrodes composed of aluminum are provided in P-type base layer exposed by said mesa processing. On the back face of the chip, a plurality of P-type emitter regions having a planar structure are formed within the surface of a flat N-type base layer, and the anode short type GTO thyristor is formed by providing the anode electrode composed of aluminum on the whole face thereof, extending over said N-type base layer and P-type emitter regions. The side face of the chip is coated with, for example, silicone resin for the insulation protection.
On the cathode electrode of the top portion of said mesas, the cathode external electrode is press-contacted, and on the anode electrode, the anode external electrode is press-contacted. The above-mentioned press-contact type device of the anode short type GTO thyristor is disclosed in KOUKAI, "Press-contact type semiconductor device", HEI, 4-137561 (12 May, 1992).
Recently, IGBT has appeared as a MOS gate drive type switching device. This device is characterized in that it has an advantage that it makes high voltage and high current operation easy, which the bipolar transistor has, and an advantage that driving is easy. As the switching device using this IGBT, a RC (Reverse Conduction) type switching device incorporating FRD (Free Wheel Diode) is known. This device is the one which connects FRD to IGBT in inversely parallel, and used in a module structure. This module is assembled by forming an electrode pattern used as given collector electrode, emitter electrode, emitter control electrode and gate electrode on the insulation substrate, and by solder mounting a plurality of IGBT back-face collectors and FRD chips, respectively, on said collector electrode pattern. The emitter on the upper face of the chip, emitter control and respective electrodes of the gate, and the corresponding electrode patterns on the insulation substrate are connected by a bonding wire, respectively.
The surface of IGBT chip mounted on the switching device of this module structure is coated with a passivation film such as polyimide, except for the bonding pad of the gate and emitter. The square shaped silicon chip of IGBT forms the collector electrode of the metal multilayer film over the whole surface of the back face of the chip in order to facilitate soldering.
For example, since the press-contact type semiconductor device composed of the conventional GTO thyristor has only one structure to press-contact one chip, there are defects described below:
(1) High current operation of the element is difficult. Namely, in order to increase the current rating of the element, it has been necessary to make the chip size large. However, if the chip size is increased, such problems are caused that in the high-speed power element such as MOS gate type switching devices of IGBT, the possibility to contain unrestorable defects which make fine processing difficult is increased to increase the failure rate. PA0 (2) Highly functionalizing and high value-adding are difficult. For example, when RC type IGBT device is produced, two different kinds of device structures, IGBT and FRD have to be produced in one wafer, thereby the production process is complicated and production becomes difficult. PA0 (1) Reliability for the Power Cycle of Bonding PA0 (2) Reliability for the Thermal Fatigue of the Soldering Layer PA0 (3) Reliability for Cooling of the Element PA0 (4) Reliability for High Temperature Operation PA0 (5) Reliability for High Voltage Operation PA0 (6) Reliability for Wiring
Furthermore, when the conventional module type semiconductor device is used to be mounted on a vehicle (for traction applications), such problems described below are caused:
It is required that semiconductor devices for traction applications have to be endurable to the power cycle of 6 million cycles at a temperature variation of 25.degree. C., but in the current technology, it can only endure 3 million cycles.
It is required that the soldering layer has to be endurable, as in said bonding, to the power cycle of 75,000 cycles at a temperature variation of 70.degree. C., but in the current technology, it can only endure about 25,000 cycles.
Conventional module type semiconductor devices are cooled on the only one face from the collector side, and cooling from the emitter electrode side is almost impossible. Therefore, in order to enhance the heat dissipation, the outside diameter of the element has to be increased. For example, in the RC type IGBT module of 1200 A-2500 V class, the outside diameter becomes 130.times.260.times.40 mm.
In semiconductor devices for traction applications, the operating temperature in use is had to be insured in the range -40.degree. C. -150.degree. C., but the resin of the case material is not endurable against such temperature.
Since the case material is a resin and easy to cause tracking, it is not endurable against high voltage operation.
Since wiring within the module is fine and complicated, there are such problems as noise generation in the gate circuit due to the wiring inductance, and generation of the spike voltage at the time of diode recovery.